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Modulator

Demodulator

Transmission Channel

Input transducer

Transmitter

Receiver

Output transducer

Carrier

EM waves (modulated signal)

Baseband signal (electrical signal)

Baseband signal (electrical signal)

Basic analog communications system

3. Modulasi Amplitudo Modulasi amplitudo ( AM = Amplitudo Modulation) : Suatu teknik modulasi, dimana amplitudo sesaat gelombang sinyal pembawa (carrier), diubah-ubah sesuai dengan amplitudo sesaat gelombang sinyal informasi.

Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal.

The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal.

Gelombang sinyal pembawa berfrekuensi tinggi, gelombang sinyal informasi berfrekuensi rendah.

Beberapa jenis /macam teknik modulasi amplitudo :

1) AM-DSB-FC: Amplitudo Modulation Double Side Band Full Carrier2) AM-DSB-SC: Amplitudo Modulation Double Side Band Suppressed Carrier3) AM-SSB-FC: Amplitudo Modulation Single Side Band Full Carrier4) AM-SSB-SC: Amplitudo Modulation Single Side Band Suppressed Carrier5) AM-VSB : Amplitudo Modulation Vestigial Side Band, yang digunakan

AM-DSB-FC : Modulator AM merupakan perangkat nonlinier. Modulator memiliki dua input dan satu output. Input pertama sinyal gelombang informasi Vm(t). Input kedua gelombang sinyal pembawa Vc(t). Output gelombang sinyal termodulasi VAM(t). Vm(t) = VmSinωmt.

Vc(t) = VcSinωct.

Persamaan Gel AM-DSB-FC :

Gelombang Sinyal Informasi

Gelombang Sinyal Pembawa (Carrier)

Gelombang Sinyal AM (termodulasi)

tfVtVFor ccc 2cos)( tfVtV mmm 2cos)(

tfftffmVc

tfVtV

givesWhich

tftfmVtfVtVgetWe

YXYXYXGiven

tftfmVtV

tftfVVtVSignalModulated

mcmcccAM

mccccAM

cmcAM

cmmcAM

)(2cos)(2cos2

2cos)(

:

2cos2cos2cos)(:

)cos(2

1)cos(

2

1))(cos(cos:

2cos2cos1)(

2cos2cos)(:

Dimana m = Indeks modulasi,

c

m

V

Vm =

Persamaan Matematis Gelombang AM-DSB-FC

c

m

V

Vm =

%

%

X 100 %

X 100 %

Untuk Modulasi AM, indeks modulasi maksimum sebesar 1 atau 100 %, jika indeks modulasi lebih besar dari 1 atau lebih besar dari 100 %, maka akan terjadi over modulasi.

AM – Percentage Modulation

Under modulated (<100%) 100% modulated

Envelope Detector

Can be used

Envelope Detector

Gives Distorted signal

Over Modulated (>100%)

AM – Percentage Modulation

Spektrum, Bandwidth dan Daya AM-DSB-FC

Spektrum AM-DSB-FC

Banwidth, Bw = 2 fm

Daya pancar totalAM-DSB-FC :

21

2mPP ct

DSB-FC EXAMPLE1. FOR AN AM DSB-FC MODULATOR, WITH CARRIER FREQUENCY OF 100 kHz, AND A MAXIMUM MODULATING SIGNAL OF 5 kHz, DETERMINE:

a) FREQUENCY LIMITS FOR THE UPPER AND LOWER SIDEBANDS

Jawab : (100 - 5) kHz TO 100 kHz = 95 kHz TO 100 kHz = LSB 100 kHz TO (100 + 5) kHz = 100 kHz TO 105 kHz = USB

b) BANDWITH OF THE MODULATED SIGNAL

Jawab : B = 2 fm = 2 x 5 kHz = 10 kHz

Jawab : (100 - 3) kHz = 97 kHz = LSF (100 + 3) kHz = 103 kHz = USF

c) UPPER AND LOWER SIDE FREQUENCIES WHEN MODULATING SIGNAL IS A 3 kHz TONE

2. FOR THE AM WAVEFORM BELOW:

DETERMINE:

a) PEAK AMPLITUDE OF THE UPPER AND LOWER SIDE FREQUENCIES

)(4

1

2minmax VV

VVV

mlsfusf

VoltVV lsfusf 4)218(4

1

b) PEAK AMPLITUDE OF THE UNMODULATED CARRIER

VoltVVVc 10)218(2

1)(

2

1minmax

c) PEAK CHANGE IN THE AMPLITUDE OF THE ENVELOPE

VoltVVVm 8)218(2

1)(

2

1minmax

Jawab :

Jawab :

Jawab :

d) COEFFICIENT INDEX

8.010

8

minmax

minmax

VV

VV

V

V

c

m

e) PERCENT MODULATION

%80%1008.0%100 xxV

Vm

c

m

%80%100218

218%100

minmax

minmax

xxVV

VVM

Jawab :

Jawab :

3. ONE INPUT TO A CONVENTIONAL MODULATOR IS A 500 kHz CARRIER WITH AN AMPLITUDE OF 20 Vp. THE SECOND INPUT IS A 10 kHz MODULATING SIGNAL THAT IS OF SUFFICIENT AMPLITUDE TO CAUSE A CHANGE IN THE OUTPUT WAVE OF .

DETERMINE:a) UPPER AND LOWER SIDE FREQUENCIES

b) MODULATION COEFFICIENT AND PERCENT MODULATION

(500 + 10) kHz = 510 kHz = USF(500 - 10) kHz = 490 kHz = LSF

Vp5.7

375.020

5.7m

%5.37%10020

5.7 xm

Jawab :

Jawab :

C) PEAK AMPLITUDE OF THE MODULATED CARRIER

d) UPPER AND LOWER SIDE FREQUENCY VOLTAGES

Vc (MODULATED) = Vc (UNMODULATED) = 20 Vp

VpmVV

VVcm

lsfusf 75.32

)20(375.0

22

e) MAXIMUM AND MINIMUM AMPLITUDES OF THE ENVELOPE

mcmc VVVVVV minmax ;

VpV 5.275.720max

VpV 5.125.720min

Jawab :

Jawab :

Jawab :

4. FOR AN AM DSB-FC WAVE WITH A PEAK UNMODULATED CARRIER VOLTAGE Vc = 10 Vp, A LOAD RESISTANCE OF RL = 10 Ohms, AND A MODULATION INDEX OF 1,

DETERMINE:

a) CARRIER POWER

b) UPPER AND LOWER SIDEBAND POWER

WR

VP

cc 5

)10(2

10

2

22

WPm

PP clsbusb 25.14

)5(1

4

2

c) TOTAL SIDEBAND POWER

WPm

PP clsbusb 5.22

)5(1

2

2

Jawab :

Jawab :

d) TOTAL POWER IN THE MODULATED WAVE

Wm

PP ct 5.7)2

11(5)

21(

22

POWER SPECTRUM:

Jawab :

Thankyou, see you next

week, ok

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